When it comes to darker skin, pulse oximeters fall short
Over the past two years, the pulse oximeter has become a crucial tool for tracking the health of COVID-19 patients.
The small device clips onto a finger and measures the amount of oxygen in a patient's blood. But a growing body of evidence shows the device can be inaccurate when measuring oxygen levels in people with dark skin tones.
A study published on Monday only adds to this concern.
Researchers analyzing pre-pandemic health data also find those measurements resulted in patients of color receiving less supplemental oxygen than white patients did.
"We were fooled by the pulse oximeter," says the study's lead author Dr. Leo Anthony Celi, who's clinical research director and principal research scientist at the MIT Laboratory of Computational Physiology.
"We were given the false impression that the patients were okay. And what we showed in this study is that we were giving them less oxygen than they needed," he says.
These sobering findings are bringing more urgency to educating patients and medical professionals about the shortcomings of the pulse oximeter — and to designing new models that can work reliably regardless of someone's skin color.
A doctor fights to get her son care
It was last September when Dr. Sandra Looby-Gordon saw how this flaw in the device could affect her own family.
Looby-Gordon, who's a physician at Boston Medical Center, found herself on the phone with a triage nurse at a Florida hospital, arguing that her own son — who was very sick with COVID-19 — needed to be admitted to the hospital.
"'Well, yeah, he is looking pretty short of breath,'" Looby-Gordon remembers the nurse responding, "'but his oxygen levels are good.'"
The nurse was basing this on the reading from the pulse oximeter clipped to his finger, but this assessment did not feel right to Looby-Gordon.
She got off the phone with the nurse and spoke with other doctors at her medical center. One of them reminded her of a 2020 article in the New England Journal of Medicine showing the pulse oximeter tends to be inaccurate in people with dark skin tones.
"On top of that, my son is — this sounds strange — but very dark, very dark complexion," says Looby-Gordon.
Sure enough, later when her son was given a more invasive test for measuring blood oxygen, it showed his oxygen levels were actually dangerously low.
He was admitted to the hospital, treated and ultimately recovered from COVID-19. But Looby-Gordon says most patients in their situation wouldn't know about the shortcomings of the pulse oximeter.
Even as a Black physician herself, she says she wasn't fully aware of how the device could be so misleading.
Research highlights device's shortcomings
If anything, the pandemic has underscored this longstanding problem with the pulse oximeter.
Research published last monthby scientists at Johns Hopkins University shows inaccurate results from pulse oximeters resulted in a failure to identify Black and Hispanic patients who were in need of COVID-19 treatments like the steroid dexamethasone and the antiviral remdesivir.
Throughout the COVID-19 crisis, people of color have experienced higher rates of hospitalization and death from COVID-19 compared to white people. Celi of MIT says it's not possible to know how much pulse oximeters have contributed to the disproportionate impact of COVID-19 on people of color, but he believes it has played a role.
And the issue points to a larger problem with how medical devices are studied and approved: "The way we evaluate medical products is primarily based on trials that involve primarily white individuals," Celi says.
FDA guidance for approving pulse oximeters says clinical trials should include at least two darkly pigmented people, or 15% of the subject pool — whichever is larger. But some doctors and scientists say that's insufficient, especially since there's such a wide range of skin tones.
Several manufacturers of pulse oximeters — including Edwards Lifesciences, Masimo and Nonin — claim that their own versions of the devices provide accurate results that do, in fact, take skin tone into account.
In a 2021 op-ed in response to the New England Journal of Medicine article, the CEO of Masimo Corp. suggested several hypotheses might account for the disparity between the results in that study and their own internal research, including sickle cell disease and circulatory problems, which disproportionately affect Black people.
Scientists look for solutions
Increasingly, scientists and engineers are working on new technologies that could revolutionize pulse oximeters so they work just as well for people with darker skin.
In an optics lab at Brown University, PhD student Rutendo Jakachira explains how a pulse oximeter works.
"If you insert your finger in this groove, the LED at the top is sending light through your finger," says Jakachira. The device can then calculate a patient's oxygenation by figuring out how much of the light was absorbed by hemoglobin in the blood.
"That's key to the problem being seen in people with dark skin, says Kimani Toussaint, a professor of electrical and computer engineering, biomedical engineering, and mechanical engineering at Brown University. "It's assuming that the only absorber of the light energy is the hemoglobin."
But in reality the skin pigmentation also absorbs the light, he says. And for people with darker skin, that can result in a reading from the pulse oximeter that overestimates the amount of oxygen in their blood.
Toussaint stands next to a table full of technology he hopes will solve the problem.
"I wouldn't even call this a device yet," he says.
Unlike current pulse oximeters, the not-quite-yet-a-device uses polarized light which isn't absorbed by skin pigmentation. If it works correctly, Toussaint says they'll partner with manufacturers to shrink it all down into a device that could be marketed.
At Tufts University, Valencia Koomson is working on tackling this problem using a different approach.
Her device uses the same kind of light as currently available pulse oximeters do, but it includes technology that can measure a person's skin tone (people with darker skin pigmentation have higher levels of melanin).
"We can send more light if there's a higher level of melanin present, so that melanin doesn't become a confounding factor that obscures our results," says Koomson, who is an associate professor of electrical and computer engineering.
Koomson, who's Black, says the story of the pulse oximeter — and ongoing efforts now to redesign it — point to the need for greater diversity in engineering and medicine.
"We're shaped by our environment and who we are and our identity," she says. "That informs what type of research goes on. It's the people who do research, who decide what research is done."
Koomson and other scientists have also been pushing the Food and Drug Administration to take steps to address the problem.
"When a patient's at home and they're not being monitored closely in the hospital, we need to make sure that those numbers are as accurate as possible so we can make clinical assessments," says Dr. Sandra Kane-Gill, president of the Society of Critical Care Medicine, which has sent two letters to the FDA about the problems with the pulse oximeter.
The agency is starting to respond.
Last winter, the FDA issued a warning that skin pigmentation and other factors could impact pulse oximeter results. Now it's funding research into the issue and will bring together expert advisors later this year to discuss how to ensure the devices are accurate for everyone.
Despite years of publications on the issue, Koomson says it's not as well-known as it should be.
She says a national legacy of racist, pseudo-scientific studies has left scientists wary of exploring physical differences between people of varied races.
"People are afraid to talk about physical differences because they won't want to appear to be discriminative," says Koomson. "But I think that we have to talk about aspects that affect people's health and have an impact on the care that they're being given."
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